1. Field of the Invention
The present invention relates to a noise reduction circuit, particularly to a noise reduction circuit in a recording medium player such as a video disk player etc.
2. Description of the Related Art
In a video signal auto-correlativity between frames is strong, but generally there is almost no auto-correlativity between noise components included in a video signal; there is a well known noise reduction circuit taking advantage of the above-mentioned phenomenon in which only the energy of noise components is lowered almost without changing the energy of a signal component by temporally averaging a video signal by every frame period.
A conventional noise reduction circuit is so constituted as shown in the following: a mixing means for mixing an input video signal and a signal to be mixing-processed at a mixing ratio corresponding to a coefficient k is provided; the output of the mixing means is delayed by one frame period with a frame memory etc., and supplied to the mixing means as the above-mentioned signal to be mixing-processed; the video signal output from the mixing means is the output signal of the noise reduction circuit.
An essential problem of a conventional noise reduction circuit is that it is effective for a still picture but for a moving picture it has a demerit of causing an afterimage etc. There is a circuit which is devised for preventing the above-mentioned defect, in which a moving picture part is detected and the coefficient k for the part is varied.
In a recording medium player such as a video disk player, when a still picture is played back, a video signal of the same frame is repeatedly read from a recording medium. In the NTSC system, however, the phase of a chrominance signal is chaned by 180 degrees every frame, so that in a recording medium player during the playback operation of a still picture, the time-axis of a video signal is shifted by an multiple of a quarter of a period of a color subcarrier for keeping the continuity of the phase of the chrominance signal in the video signal. Therefore if the noise reduction of a video signal obtained from a recording medium player is performed with a conventional noise reduction circuit, a phase difference is caused between the phase of the luminance signal in the input video signal and that of the luminance signal in the video signal read out from a memory as a signal to be mixing-processed by the shift of the time-axis performed for keeping the continuity of the phase of the chrominance signal. Since the luminance signals are averaged containing the phase difference as it is, there has been a problem that a played back picture is blurred.